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Wire and Wireless Communication Applications Chap. 8
1 frame period
From
station
A
From
station
B
From
station
C
From
station
E
From
station
F
From
station
G
From
station
H
Preamble
Data to
station
A
Data to
station
E
Data to
station
G
Data to
station
H
Guard
time
Carrier sync
and bit sync
Identification
and addressing
Figure 8–14
Typical TDMA frame format.
Another multiple-access method that is similar to TDMA is the ALOHA technique
[Lam, 1979]. Here the multiple users send bursts of data, called packets, whenever they
wish. When two or more of the bursts overlap in time, there is a collision. When a collision
occurs, the users involved retransmit their packets after a random time delay, and, hopefully,
a second collision will not occur. If it does occur, the retransmission process is repeated until
each party is successful. This technique has the advantage of being relatively inexpensive to
implement, but it will not work if there is heavy traffic loading on the satellite, in which case
the satellite becomes saturated with colliding packets, and the collisions can be avoided only
by stopping all new transmissions and increasing the random delay required before
retransmission. A more elaborate technique is called slotted ALOHA. With this method, the
packets are transmitted at random, but only in certain time slots. This avoids collisions due
to partial packet overlap.
Very small aperture terminals (VSATs) have become popular with the availability of
Ku-band satellites and with the recent advances that have made low-cost terminals possible.
“Very small aperture” implies that these systems use Earth-terminal antennas that are relatively
small (about 1 or 2 m in diameter). Solid-state power amplifiers (1 to 2 W), low-cost
frequency converters, digital processing, and VLSI circuits have made VSATs feasible. The
objective of VSAT systems is to provide low-cost data and voice transmission directly to users
such as auto dealerships, banks (automatic teller machines), brokerage firms, pipeline companies
(monitor and control), hotels and airlines (reservations), retail stores (data networks), and
corporations (point-to-multipoint transmission). Typically, VSATs offer high-quality transmission
(BERs of less than 10 -7 for 99.5% of the time) at data rates from 100 bs to 64 kbs
[Chakraborty, 1988; Maral, 1995]. Many users share a single satellite transponder via SCPC,
TDMA, or CDMA so that the user’s cost can be substantially less than that for the same type